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    <title>Document</title>
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<body>
    <!-- 基本数据类型：undefined number boolean string null -->
    <script>
    var a;   //undefined
    var b = 20; //number
    var c = true;//boolean
    var d =false;//boolean
    var str = "hello";//string
    var str_one ='good';//string
    console.log(typeof a);  //typeof -->识别一个数据类型
    console.log(typeof b);
    console.log(typeof str);
    console.log(typeof str_one);
    console.log(typeof c);
    console.log(typeof d);
    </script>
</body>
</html>